The big water in this pack comes in two kinds and you run them differently. Lake Pend Oreille, Priest Lake, Coeur d'Alene Lake and Payette Lake were lakes before anybody built a dam, and their shorelines sit where they have always sat. American Falls Reservoir, Dworshak Reservoir, Palisades Reservoir and C J Strike Reservoir are pools, and a pool stands at whatever level somebody is holding it at that week. Wind matters on both. On the second kind the calendar matters as well, because the water you launched onto in June is not the water that is there in September.
The pack is called lakes-idaho and it carries 2,271 charted water bodies. Of those, 1,243 are named, and 113 of the named lakes are larger than a square kilometer. The largest the pack carries is Lake Pend Oreille at 372.4 square kilometers, which is 92,022 acres. The twentieth largest is Payette Lake at 20.4 square kilometers, or 5,041 acres. Under all of them the pack carries depth, and that depth is built from state elevation surveys rather than from a nautical chart, because nobody makes a nautical chart for a reservoir.
Every area here is measured off the chart polygon itself, not copied from a sign at the ramp. These are the ten biggest the pack carries.
A list is hard to hold in your head, so here is the shape of it. Lake Pend Oreille covers more water than the smallest ten of those twenty put together. It is close to three and a half times the size of Coeur d'Alene Lake and better than eighteen times the size of Payette Lake. Bear Lake at 281.6 square kilometers and American Falls Reservoir at 222.5 are the only other two over 200, and after them the drop is steep. Lake Koocanusa, fourth on the list, is 115.9.
Further down, the gaps close right up. Lake Cascade and Priest Lake are 1,606 acres apart, which on water of that size is near enough to call the same lake twice. Blackfoot Reservoir and Dworshak Reservoir sit 2.3 square kilometers apart. Brownlee Reservoir and Hebgen Lake are within 123 acres of each other, so if somebody tells you one is the bigger water they are talking about something other than surface area.
The packs are cut to a box rather than to a state line, so water close to an edge is carried in more than one pack. If what you run sits near a line, the neighboring lake packs are Washington, Oregon, Nevada and Utah, and it costs nothing to have the second one on the phone.
| Lake | Charted surface area |
|---|---|
| Lake Pend Oreille | 372 km² (92,022 acres) |
| Bear Lake | 282 km² (69,585 acres) |
| American Falls Reservoir | 222 km² (54,981 acres) |
| Lake Koocanusa | 116 km² (28,639 acres) |
| Coeur d'Alene Lake | 108 km² (26,687 acres) |
| Lake Cascade | 101 km² (24,859 acres) |
| Priest Lake | 94 km² (23,253 acres) |
| Blackfoot Reservoir | 68 km² (16,803 acres) |
| Dworshak Reservoir | 66 km² (16,235 acres) |
| Palisades Reservoir | 56 km² (13,813 acres) |
| Brownlee Reservoir | 50 km² (12,454 acres) |
| Hebgen Lake | 50 km² (12,331 acres) |
The twelve largest named lakes the Idaho pack carries, by charted surface area, measured from the chart polygon itself. The pack carries 2,271 charted water bodies in all, 1,243 of them named, and 113 named lakes larger than one square kilometer.
A lake is not the sea. The sea gets surveyed by hydrographic launches and the result becomes a NOAA chart with soundings on it. Nobody does that for Lake Cascade. That is the reason a reservoir usually arrives on a screen as a blue shape with a name floating in the middle of it and nothing underneath.
Helm builds inland depth from state elevation surveys instead, and takes the outline of the water from the National Hydrography Dataset. The elevation survey is a measurement of the ground, including the ground that happens to have water standing on it, and that is what puts contours and a depth number on Priest Lake or Lake Lowell. The hydrography data is what says where the water is in the first place, which is how 2,271 separate bodies get onto the chart at all.
The practical difference is small to describe and large on the water. A blue shape tells you that a lake is there. Water with contours on it is something you can lay a line across and read against the draft of your own boat before you back the trailer down. Draft shading and routing work the same way inland as they do on the coast, and those subjects have their own pages.
Eight of the twenty largest named waters the pack carries have the word Reservoir in the name. That word is doing real work. A reservoir has a level somebody chose, and the level the day you go depends on snowpack, irrigation orders and what the dam is being asked to do that month.
Here is the part that makes contours usable anyway. The bed does not move. A contour describes the shape of the bottom, and the bottom in a cove on American Falls Reservoir in September is the same bottom that was there in June. What changed is how much water is standing on it. So the contour is still in the right place, and the number you need alongside it is the pool level the operator publishes for that day, set against the level you last ran the lake at.
Once the pool comes down, the old river or creek channel is the line that keeps carrying you, and it is visible on the chart as the deep lane running through shallower ground. Follow it out of the arm the way it was cut instead of the way the open water looks, and enter your draft so the shoulders either side of it shade before you get to them. On a drawn down pool, the launch itself is worth a look on the chart first, because the ramp that ended in water at full pool ends in a walk at low pool.
None of this is peculiar to one lake. It is how a pool behaves, and on a pack where Dworshak Reservoir, Palisades Reservoir, Brownlee Reservoir and Lake Walcott are all in the top fifteen by area, it is worth reading a contour with the level in mind every time.
Named water gets the attention. The count that matters on a Saturday when the big lake is busy is the whole 2,271, of which 1,243 carry a name and 1,028 do not. Only 113 of the named lakes clear a square kilometer. The rest are smaller than that, which is what most water anywhere actually is, and the pack carries them the same way it carries Lake Pend Oreille.
Mud Lake at 26.6 square kilometers, Henrys Lake at 24.9 and Island Park Reservoir at 30.4 are all in the top twenty by area while being a fraction of the size of the headline lakes. A boat that fits them is a different boat, and a chart that shows the bottom on them is worth more than one that shows the bottom on the big water only.
Service tends to quit before the pavement does on the road in to these. The chart pack goes onto the phone at home over wifi, and after that the chart, the depth shading, a route drawn across a cove and the measure tool all keep running at a ramp with no bars. Spots you save and the track from the day come home with you, so a contour you found by accident in August is sitting on the chart the next spring.
Inland lake depth comes from state elevation surveys rather than from nautical charts, and a reservoir sits at whatever level it is being held at that week. Treat the shape and the depth as a guide, keep a lookout, and verify locally.
Depth data. Lake Pend Oreille is the largest named water the Idaho pack carries, 372.4 square kilometers or 92,022 acres, and it comes with contours and a depth reading on the water itself rather than a plain blue outline. The depth is built from state elevation surveys, since there is no NOAA chart for an inland lake. Enter your draft once and the water your boat cannot cross shades on the chart, and it all keeps working once the pack is on the device and you are out of cell range.
Yes. Both are carried in the Idaho lake pack, along with 111 other named lakes over a square kilometer. Coeur d'Alene Lake measures 108.0 square kilometers, or 26,687 acres, and Priest Lake measures 94.1 square kilometers, or 23,253 acres. One thing to know about pack boundaries: they are cut to a box rather than to a state line, so water sitting near an edge is carried in more than one pack. The neighboring lake packs are Washington, Oregon, Nevada and Utah.
Read the contours as the shape of the bottom, because that is what they are and the bottom does not move when the pool does. American Falls Reservoir is 222.5 square kilometers at the chart polygon, 54,981 acres, and the third largest water the pack carries, so there is a lot of flat ground that goes dry at low pool. Check the level the dam operator publishes for the day you go and set it against the level you last ran the lake at, then follow the old river channel on the chart out of the arms instead of running the open water on sight. With your draft entered, the shoulders either side of that channel shade before you reach them.
Helm Boating